-9~ 
reflected and scattered back to space, This leaves as the amount of 
solar radiation absorbed by the atmosphere, ground and ocean an 
average of 1600 cal em7? day71 over the area intercepted by the 
earth. This area is one quarter of the area of the sphere, so an 
average of 400 cal em~2 day-l is received over the area of the earth's 
surface. Since the radiation emitted by the earth must practically 
balance that absorbed, a square centimeter of the earth's surface 
must radiate an averare of 400 calories per day of long-wave radiation, 
which is the amount emitted by a Square centimeter of an ideal radi-~ 
ator at 241°A or -26°F, 
The behavior of bodies toward long-wave radiation may be very 
different than toward short-wave radiation. It has already been 
mentioned that light penetrates rather far into water, which is very 
opaque for long-wave radiation. The reflecting vower or albedo of 
clouds and of snow-cover, including most sea-ice, is between 0.7 and 
0.8 for solar radiation, but these surfaces are practically ideal — 
radiators for the long-wave radiation they emit at terrestrial tem- 
peratures. This is part of the explanation of the lower temperatures 
of snow or ice surfaces than of bare ground or water, and hence of 
the greater cooling of the air over them. 
The absorption of radiation of a given wave length by a thin 
layer of semi-transparent material normal to the beam is equal to 
the absorption coefficient times the amount of radiation entering the 
layer. It follows for instance that as a narrow wave-length band of 
solar radiation passes through uniform sea water, the first centimeter 
of water absorbs a certain fraction of the energy passing through the 
sea surface, the second centimeter absorbs the same fraction of what 
is left, and so on. So the first centimeter absorbs more than the 
second, and the second more than the third, The logarithm of intensity 
